1. Field of the Invention
The present invention relates to a process for producing non-proteinogenic L-amino acids by direct fermentation of microorganisms, and to L-amino acids obtained by the process.
2. The Prior Art
Non-proteinogenic amino acids are amino acids which are not used in nature as building blocks for protein biosynthesis and as a result may be clearly differentiated from the 20 proteinogenic amino acids. They are preferably β-substituted L-alanine derivatives.
Non-proteinogenic amino acids are compounds of interest, for example, for the preparation of pharmaceuticals and agricultural active compounds. They can, as active compound or as a part of an active compound imitate, in a type of molecular mimicry, the structure of natural amino acids and as a result, for example, in receptor interactions cause a modulation of the natural reaction. In addition, they can serve quite generally as synthesis building blocks as chiral compounds in the context of the “chiral pool”.
Previous production processes for non-proteinogenic amino acids in enantiomerically pure form are generally based on complex syntheses which generally only permit access to a defined compound. Only a few processes enable different compounds to be produced by simple replacement of a starting material.
In most cases chemical syntheses are involved which themselves mostly start from the beginning from chiral building blocks or are followed by a racemate resolution.
Alternatively, some enzymatic processes are described. Thus, using transaminases, various non-proteinogenic amino acids can be prepared from α-keto acids using L-glutamic acid as amino donor. A different process utilizes hydantoinases in combination with carbamoylases. However, enzymatic processes are also cost-intensive, since the corresponding enzymes must be provided and these have only a limited life as catalysts (Rehm et al., Biotechnology 1996; Vol. 6, pp. 505–560).
In contrast, processes for producing non-proteinogenic amino acids by direct fermentation of microorganisms would be particularly simple and expedient. However, such processes have the risk that the non-proteinogenic amino acid produced interferes with the metabolism of the natural amino acids and thus growth inhibition occurs. Previously, within this subject area, a process for the direct fermentation of D-amino acids has been disclosed (WO98/14602). This application describes the production of D-amino acids by recombinant microorganisms into which a D-amino transferase gene and an L-amino deaminase gene have been introduced. Furthermore, Saito et al. (Biol. Pharm. Bull. 1997, 20: 47–53) described the production of the plant non-proteinogenic amino acid L-pyrazolylalanine by expressing plant genes in Escherichia coli. The yields, however, are too low for commercial production, at <1 g/l, and the costs, with the described use of L-serine as starting material, are very high.